Why should everything have to have a clear and stated purpose?
Because that is the way the Government decrees our (MoD) financial planning works. (1) There shall be a Navy: (2) This is what it will do: (2) This is what we (the Navy) need to do it. (In descending order of decision maker seniority).
It is there like all militaries to do whatever is needed when asked.
Correct, although it is told. The Navy chiefs are in turn paid to tell politicians when this cannot be reconciled with (2) above. They seldom do.
The real world is not black and white and quantifiable no matter how much people will try to stick it in boxes.
Correct again, but given the above, some are paid (not a lot) to make decisions of what event is more likely to occur. Very often they are wrong, which is why we need a greater degree of built-in flexibility.
Most would be quite surprised how low a level many of these decisions are made at, and how small the RN's input is. Perhaps worse, the "box-ticking" exercise you mentioned was ditched about 25 years ago. At least it was a laid down process with robust procedures. Today, it's more like making it up as you go along.